The precision grinding and polishing of hard materials, such as optical glass, typically includes several steps that progressively improve surface characteristics, particularly the figure or curvature of the surface. Starting from a blank that approximates the desired shape, material might be removed first by cutting, then by grinding and finally by lapping. Cutting typically is used to establish an approximate profile, while grinding refines the shape and lapping establishes the final figure and finish. Each stage removes less material but does so more precisely. Equally important, each stage removes stresses and damage to the surface from prior operations.
Precision grinding typically moves a rotary abrasive against the surface in minute increments. A massive support structure is used for rigidity, and a fine advancing mechanism for precision. The abrasive locally fractures and removes material, but must be controlled to prevent damage from excessive heat or pressure.
High velocity jets including suspended abrasives have been used for removing one material from the surface of a different material (e.g. cleaning and sandblasting), and for precision cutting of glass and other hard materials (e.g. waterjet cutting). In cleaning and sandblasting applications, nozzles are provided primarily for aiming the particle stream rather imprecisely in the desired direction for the purpose of removing surface contamination such as rust and paint. In precision cutting operations, on the other hand, the nozzle is aimed more precisely and its location is controlled for movement relative to the work surface accurately to cut the desired contour. The objective in cutting normally is to make sure the stream passes entirely through the work.
Prior art grinding and polishing techniques must be limited in speed to prevent internal stress build up and other damage, particularly when applied to large optics for observatories, and the like, that require perfection measured in the wavelengths of light. The speed of the abrasive relative to the work surface is limited by the mass of the abrasive tool or lap that supports the abrasive and applies it to the surface.
Previous approaches also suffer from the relatively unyielding pressure between the abrasive and the work surface. High spots can result in substantial forces and heat generation, certainly against a hard abrasive, but also against the softer materials employed in a lap. Internal stresses from the resulting heat and strain have long term deleterious effects. Previous approaches also suffer from the stringent requirement to rigidly support the workpiece being shaped, without significantly distorting it. This necessitates strain free holding fixtures which workpiece during the grinding and polishing process. If the holding fixture is not strain free, the shaped surface may "spring" when the workpiece is removed from the holding fixture, resulting in a misshapen surface. Furthermore, the holding fixtures of the prior art need to tightly constrain the position of the workpiece against lateral forces generated by the grinding and polishing steps, since even slight shifting of the workpiece during grinding or polishing will result in a misshapen surface.